As semiconductor devices have scaled to smaller and smaller dimensions, the gate dielectric thickness has continued to shrink. Although further scaling of devices is still possible, scaling of the gate dielectric thickness has almost reached its practical limit with the conventional gate dielectric material, silicon dioxide. Further scaling of silicon dioxide gate dielectric thickness will involve a host of problems: extremely thin layers allow for large leakage currents due to direct tunneling through the oxide. Because such layers are formed literally by a few monolayers of atoms, exacting process control is required to repeatably produce such layers. Uniformity of coverage is also critical because device parameters may change dramatically based on the presence or absence of even a single monolayer of dielectric material. Finally, such thin layers form poor diffusion barriers to impurities.
Realizing the limitations of silicon dioxide, researchers have searched for alternative dielectric materials that are thicker than silicon dioxide and yet still produce the same field effect performance. This performance is often expressed as “equivalent oxide thickness.” Although the alternative material layer may be physically thick, it has the equivalent electrical effect of a much thinner layer of silicon dioxide (commonly called simply “oxide”). In the most recent technology node devices silicon dioxide has been replaced with a SiON. However, even SiON will have to be replaced by high-κ dielectrics to reduce leakage as the equivalent oxide thickness is reduced. Some films currently being investigated include deposited oxides, nitrides, and oxynitrides such as HfOx, HfSiO, HfSiON, AlON, and AlZrO. Manufacturable processes for incorporating these materials are needed.